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Memory ranges for PMEM are described by either an NFIT
(NVDIMM Firmware Interface Table, see NFIT_ACPI), a
non-standard OEM-specific E820 memory type (type-12, see
X86_PMEM_LEGACY), or it is manually specified by the
'memmap=nn[KMG]!ss[KMG]' kernel command line (see
Documentation/kernel-parameters.txt). This driver converts
these persistent memory ranges into block devices that are
capable of DAX (direct-access) file system mappings. See
Documentation/nvdimm/nvdimm.txt for more details.